This invention relates to an arrangement for paralleling multiple TV transmitters to obtain high power output.
It is well known to parallel output stages of a power-handling device to increase the output power. Thus, the output power can be doubled (3 dB) by combining two output stages, trebled (4.7 dB) by the use of three output stages, and so forth. The paralleling of outputs in addition to providing higher power also provides improved reliability, in that operation at reduced power continues if one of the output stages fails.
In television broadcasting, reliability of the transmitter is economically important. Consequently, it is desirable to operate a plurality of power stages which are combined to produce the desired output signal. In order to further enhance the reliability of a broadcast transmitter, the prior art includes the signal processing and control stages associated with each power output stage in the paralleled arrangement. Thus, each power stage and its associated level adjusting (ALC) system, attenuators, linearity correctors, filters and the like are considered as one unit, and a plurality of such units ar paralleled. In such an arrangement, failure of one of the control or signal processing circuits associated with a power output stage merely causes reduced output power rather than complete failure.
Among the signal processing circuits currently in use are surface acoustic-wave (SAW) filters. These filters are useful for forming vestigial sidebands because of their small size, repeatability from unit to unit and the like. SAW filters have the disadvantage, however, that the signal transit time and consequently the phase-shift are temperature-dependent. While changes in phase attributable to SAW filters may be minimized by the use of ovens, it has been found that the operation of SAW filters in parallel gives rise to differential phase shifts between the two channels. Such phase shifts may cause large changes in the output power produced by the combined output stages and increases the power dissipated in the power stages themselves. Consequently, the parallel approach has proved to be deficient.
Another approach in the prior art provides reliability in a different manner. In this arrangement, the power stages are paralleled and their combined input is coupled by means of a selecting switch to one of a plurality of control and signal processing arrangements. Thus, for example, two separate and independent signal processing arrangements may be provided, one of which is normally in use and one of which is in standby. Upon the occurrence of a failure in the operating signal processing arrangement, the selecting switch is thrown to select the standby unit, and operation continues as before.
However, in such an arrangement the correction of linearity by predistortion of the signal is comprised, because the distortion generated by each of the paralleled output stages is different in amplitude and phase, and the resulting combined distortion is difficult to predict. Consequently, the design of the linearity correctors becomes complex and in practice a compromise predistortion must be accepted. The compromise distortion is undesirable in that if one of the output stages fails, the predistortion is no longer correct for the single output stage alone. In this case, the failure mode causes increased distortion in addition to the drop in output power. An additional disadvantage of this arrangement is apparent when a closed-loop automatic level control (ALC) is desired. For proper operation, the power control attenuator must proceed the IF linearity and incidental phase correction circuits so that the overall system linearity is not changed by changes in power level. This arrangement requires that ALC loop control circuits sample both high power amplifier outputs and provide control signals to both exciters. Hence, the two high power amplifier controls are not independent and reliability is compromised.